The present invention pertains to a process for preparing polymers and copolymers based on vinyl chloride by mass polymerization of a monomer composition based on vinyl chloride. It also pertains to the polymers and the copolymers based on vinyl chloride prepared according to the said process.
The mass polymerization of a monomer composition based on vinyl chloride first progresses in the liquid monomer composition. Due to the insolubility of the polymer or copolymer in the monomer composition, the reaction product will separate in the solid state during the polymerization in the form of particles which are maintained in suspension in the liquid monomer composition by subjecting the reaction mixture to agitation. If the polymer or copolymer content; on a weight basis, in the reaction mixture reaches a value on the order of magnitude of 15%, its consistency becomes comparable to that of agitated curdled milk. This consistency increases until the monomer composition will no longer form a continuous liquid phase, which occurs when the polymer or copolymer content; on a weight basis, in the reaction mixture reaches a value on the order of magnitude of 25%, a value beyond which the reaction mixture is in the powdered state. The reaction mixture is maintained in this state by continued agitation, which prevents the reaction product from lumping together, until the polymer or the copolymer is obtained in the powdered form at the end of the polymerization, after degassing of the nonreacted monomer composition.
The term "monomer composition based on vinyl chloride"; or "monomer composition" for short, is defined in the present description as vinyl chloride alone or mixed with at least one other monomer which is copolymerizable with vinyl chloride. The said monomer composition based on vinyl chloride contains at least 70 wt. % vinyl chloride. Especially vinyl acetate and the low-molecular-weight olefins, e.g., ethylene and propylene, can be mentioned as monomers which are copolymerizable with vinyl chloride.
The mass polymerization of a monomer composition based on vinyl chloride is generally carried out in the presence of at least one polymerization initiator which is soluble in organic solvents and generates free radicals, e.g., an organic peroxide or an azo compound.
When the intended rate of conversion of the monomer composition is reached, the polymer obtained is subjected to a degassing treatment, which is carried out while agitating and is intended to separate the nonreacted monomer composition from the polymers or copolymers.
The powdered polymer or copolymer obtained after degassing of the nonreacted monomer composition is brought to atmospheric pressure generally with an inert gas such as nitrogen or water vapor before it is exposed to the open air, which is generally followed by a screening operation.
The screening operation, which is carried out industrially as a continuous process, serves to separate the noble product, defined as the product passing through a screen with a given mesh size, selected as a function of the grain size distribution of the polymer or copolymer and the intended use, and the coarse product, which essentially consists of resin particle agglomerates, is of a lower commercial value and is defined as the oversize on the said screen.
It was previously proposed that the preparation of polymers and copolymers based on vinyl chloride by mass polymerization or mass copolymerization should be carried out in two phases according to a process consisting of the polymerization or copolymerization operations, ensuring high-speed stirring of the reaction mixture during the first phase, until the conversion rate of the monomer composition reaches a value on the order of magnitude of 7% to 15%, preferably between 8% and 12%, after which the stirring speed during the second phase of the reaction is reduced to the lowest possible value, however, still remaining sufficient to ensure good heat exchange within the reaction mixture, and this speed is maintained until the end of the reaction. According to these techniques, the two phases, the first of which is carried out with high-speed turbulizing agitation and the second with slow agitation, are carried out either in a single reactor or in different reactors, in which case the said phases, the first of which is now called the prepolymerization phase and the second is called the final polymerization phase, are carried out in appropriate devices called prepolymerizers and polymerizers, respectively. According to these techniques, the second phase is carried out in a reaction mixture formed either solely by the monomer composition/polymer from the first polymerization phase or by the said monomer composition/polymer and by a supplementary monomer composition based on vinyl chloride, which is identical to or different from the composition used during the first phase, as well as one or several polymerization initiators.
These processes and their variants were described in detail in French Patents and Certificates of Addition Nos. 1,357,736, 83,377, 83,383, 83,714; 1,382,072, 84,958, 84,965, 84,966, 85,672, 89,025; 1,436,744, 87,620, 87,623, 87,625, 87, 626; 1,450,464; No. 1,574,734, and No. 1,605,175. Improvements of the polymerizers used to carry out the said processes were described in French Patent No. 73 05537, published as No. 2,218,350 and No. 75 32124, published as No. 2,328,722, as well as in French Patent Application No. 85 05429, published as No. 2,580,192.
These processes and their variants make it possible to prepare, in a reproducible manner, polymers and copolymers based on vinyl chloride which after screening possess interesting characteristics in terms of high apparent density, mean particle diameters adjustable to need and the low weight percentage of fine particles.
However, the preparation of polymers and copolymers based on vinyl chloride according to these processes requires the use of two agitators of different types, one for high-speed turbulizing agitation during the first phase and another for slow agitation during the second phase.